D/A converters function to produce at their outputs an analog level signal corresponding to the value of a digital word signal applied to their inputs D/A converters contemplated for use in practicing the invention are, normally, periodically "initialized" because the D/A converter output and some of its internal nodes may be at, or assume, undesirable levels. This is particularly true of a D/A converter which includes a capacitive voltage divider ladder network since charge may build up at various points of the ladder network resulting in offset voltages at various nodes. It is therefore necessary to periodically initialize the output, the input and the internal nodes of the D/A converter before, or after, each new digital word signal is applied to the D/A converter. Typically, the D/A converter is initialized just prior to the application of a new digital word to the input of the D/A converter. Where digital word signals are cyclically (or periodically) applied to a D/A converter, each data conversion cycle (T.sub.D) includes an initialization interval (T.sub.I) followed by an analog conversion interval (T.sub.C). During the T.sub.I, the output of the D/A converter is driven to a reference level and, during the T.sub.C, the output of the D/A converter is driven to a level determined by the value of the inputted digital word.
It is known in the art to connect a filter, such as a switched capacitor filter, to the output of a D/A converter in order to smooth the wave shape (i.e. remove sharp discontinuities). By way of example, a known switched capacitor filter samples the output of a D/A converter during M (e.g. 16) equal subintervals or time units, where M is an integer, and during one of the M sample time units (normally, the first or last) the output of the D/A converter is set at a reference level amplitude. Typically, the switched capacitor filter includes a sample and hold input section for sampling the output of the D/A converter and storing the levels of the sampled signals, and an integrator, which integrates, or smoothes, the samples. The signal produced at the output of the integrator may then be processed further by succeeding filter stages.
The periodic initialization of the D/A converter, although necessary, introduces undesired, erroneous, or noise components into the signal produced at the D/A output. Each time the D/A converter is initialized, its output is switched, at the onset of initialization, from a level corresponding to an input signal to a level corresponding to a reference voltage and, at the termination of initialization, the output of the D/A converter is again switched from the reference level to a new signal level. Thus, due to the need for initializing the D/A converter, its output is switched twice during a short interval of time, producing discontinuities and high frequency noise components which are then applied to the filter. The introduction of these discontinuities and high frequency noise components into the system is undesirable since it requires additional filtering, which means added complexity and cost. Also, additional filtering delays the processing of the signal.